Many applications call for the sensing of a condition, such as pressure, acceleration, torque and force, at a plurality of locations. By way of example in the automotive environment, electro-hydraulic brake or EHB systems generally have six locations that require sensing of the fluid pressure. Pressure sensing is required at each wheel for closed loop brake force control, at a location to sense driver input and at the pressure accumulator to sense system reserve pressure. Hydraulic sensing points are all routed through the hydraulic control unit or HCU having a system controller, i.e., microprocessor, so that there exists one member at which all different hydraulic circuit pressures are sensed. The provision of six discrete pressure sensors with full conditioning electronics results in suitable operation; however, it also results in a total pressure sensor cost which is higher than desirable compared to the remaining system component costs.
In copending, coassigned application Ser. No. 10/036,719, the subject matter of which is incorporated herein by this reference, a low cost system and method is disclosed and claimed in which a plurality of bridge type sense elements are connected to an ASIC (application specific integrated circuit) with the output of the sense elements sequentially connected to a common signal conditioning circuit path by analog multiplexing. The sense element signal is conditioned by the signal conditioning circuit of the ASIC to provide partial conditioning comprising basic calibration data. Complete characterization data provided at the time or manufacture for all the sense elements is stored in non-volatile memory of the ASIC and is transferred to a host controller, e.g., microprocessor, upon command to enable the host controller to perform appropriate mathematical operations to provide the additional amount of compensation required to complete the signal conditioning.
The ASIC also includes diagnostic fixed test bridges for diagnosing ASIC faults and a signal diagnostic path for diagnosing sense element and sense element connection faults.
The specific sense elements disclosed in the application are formed of individual strain bridges known in the art, such as silicon resistor bridges bonded to a diaphragm through glass material fired at high temperatures and adapted for placement in a fluid pressure port. When pressurized, fluid in the pressure port causes the diaphragms and bridges to undergo elastic strain. Because the resistors in the bridges are made of silicon, they exhibit a piezoresistive effect exhibiting a change in resistance when subjected to strain. By applying a voltage to the bridges, a small voltage change results at the output of the bridges.
Although the above system provides diagnostic resolution suitable for certain applications, there is a need for a higher degree of resolution in other applications, particularly in some of the most safety critical applications. For example, the fixed test bridges provide information on specific discrete points of the signal path, in the example shown, two discrete points. While these are effective for a majority of problems, there is a possibility that some errors in the signal path could go undetected.
Further, with respect to diagnosing sense element and sense element connection faults, the degree of resolution which is possible to obtain is limited due to inherent tolerances of the sensors, signal dependency on the sensed conditions such as pressure and temperature and other behavior associated with strain gauges.